Method for producing a cam that can be placed on a hollow shaft to form a camshaft

ABSTRACT

A process is provided for the production of a cam to be joined to a hollow shaft to form a camshaft. In order to make the cam suitable for stable long-term operation on the camshaft, the internal compressive stresses achieved in the cam track before the joining of the cam to the camshaft being so high that the tensile stresses resulting from joining are permanently overcompensated, it is provided that the material selected for the cam is a steel in which the carbon can be dissolved easily as cementite in the pearlite and martensite or is in the form of a fine composite carbide. The cam, which is composed of a hardenable steel, should be fully hardened and, during tempering in the hardening operation, the basic hardness should be set in a range of between 25 and 40 HRC. The cam should then be edge-zone-hardened in a two-stage heating process by induction hardening at a medium frequency of 10-35 kHz, after the preheating stage of the heating process, the introduction of heat being interrupted for 0.3-1.5 seconds. After the action of the preheat, the cam track should then be heated in such a way by means of medium frequency that an edge-zone hardening depth of between at least 0.5 mm and at most 2.0 mm relative to the cam base circle and the cam flanks and at most 2.2 mm relative to the cam tip is obtained in the middle of the track, after which the cam is sprayed with a quenching medium immediately, within a matter of milliseconds of the end of the heating process.

The invention relates to a process for the production of a cam to bejoined to a hollow shaft to form a camshaft in accordance with theprecharacterizing clause of Patent Claim 1.

Weight-optimized camshafts can be produced economically by joining camsto a tube in a form- and force-locking manner. As regards functionalreliability, the cams must have both a high adhesion on the tube foroperational reliability even when accepting high engine torques and havea hardened track, and, for reasons of wear resistance, the latter musthave sufficient internal compressive stresses after the joining of thecams. This applies particularly to the cam flank and, to a limitedextent, also to the cam tip in valve timing systems with roller camfollowers. Where internal compressive stresses are absent or internaltensile stresses are present in the cam track, additional tensilestresses are caused during engine operation by rolling-contactoperations, and these stresses can lead to overloading of the materialin the track, giving rise to fatigue or disintegration of the latter.

A method of the generic type is known from DE 44 20 C2.

The object on which the invention is based is to develop a method of thegeneric type in such a way that the cam is suitable for stable long-termoperation on the camshaft, the internal compressive stresses achieved inthe cam track before the joining of the cam to the camshaft being sohigh that the tensile stresses resulting from joining are permanentlyovercompensated.

According to the invention, the object is achieved by the features ofPatent Claim 1.

Thanks to the invention, reproducible production of a cam with optimumfunctional properties with regard to secure retention of the cam on thehollow shaft under operational load after joining and to adequateintrinsic robustness in relation to the mechanical loads acting on itduring engine operation is achieved by the interaction of a number ofcriteria. The selection of a cam material with a very high yieldstrength of at least 450 N/mm² is absolutely decisive here for theachievement of the properties mentioned, these properties emerging fromthe heat treatment of the cam blank. Such cam materials include thehardenable steels 100 Cr 6, Ck 67, C 60, C 70 etc. To achieve therequired minimum yield strength, these steels should be heat-treatedbefore edge-zone hardening but can also be used in the hot-pressedcondition (BY condition). The material stressed in accordance with theinvention provides a high-strength structure which, owing to the largedifference in yield strength relative to the tube, which is composed ofthe material St 37, St 52 or the like, for example, gives high camadhesion during the subsequent joining of the fully-treated cam thanksto the elastic springback of the cam towards the tube, which is expandedplastically due to internal high pressure. In the case of heattreatment, it is necessary, in addition to making a suitable selectionof steel material, to temper the steel in such a way that a basichardness in a range of between 25 and 40 HRC is obtained since, below 25HRC, unwanted plastic expansion of the cam during joining would occurand, above 40 HRC, stress cracks would occur in the cam track duringsubsequent edge-zone hardening or joining. The cam is then gentlyedge-zone-hardened by induction hardening in a two-stage heating processusing an annular or shaped inductor at a medium frequency in a range of10-35 kHz. Gentle edge-zone hardening means that the cam is preheated inthe first stage of the heating process for up to 1.5 seconds at a powerof about 40 kW, after which the introduction of heat is interrupted for0.3-1.5 seconds. This pause avoids overheating effects in the structure,especially in the region of the track, and excessively high edge-zonehardening depths, which would normally occur after a second heatingstage that immediately followed preheating, and a very gentle continuousdrop in hardness from the edge zone to the core, which is close to thecam bore and is not edge-zone-hardened, is achieved. Such a drop inhardness is essential since otherwise the edge zone would be susceptibleto cracking. The hardness in the region of the track is 60 HRC, forexample, and decreases to 30 HRC in the region of the core. Moreover,the formation of a coarse martensite structure with soft residualaustenite that occurs if further heating follows directly, i.e. if thereis a second heating stage that continues directly after preheating, isavoided, leading to low internal compressive stresses in the structure.

Following the action of the preheat, the cam track is heated once moreby means of medium frequency at increased power relative to that forpreheating (second heating stage), giving a temperature above theaustenitization temperature. The core structure is not affected by thisprocess. Owing to the second heating stage in conjunction with theadvantageous effects described of the preceding pause between the twoheating stages of the heating process, it is now possible to establishan edge-zone hardening depth in the middle of the cam track of betweenat least 0.5 mm and at most 2.0 mm relative to the cam base circle andthe cam flanks and at most 2.2 mm relative to the cam tip. It should bestated here that the figure of 0.5 mm in the middle of the track relatesto a cam on which there is no need for a grinding allowance. Once theedge-zone hardening depth has been established and the heating processis complete, the cam is then immediately sprayed with an aqueousquenching medium (within a matter of milliseconds), as a result of whichthe austenitic structure of the cam is transformed completely intomartensite. Very high internal compressive stresses arise in thestructure during this transformation. The formation of other types ofstructure that produce only low internal compressive stresses is avoidedowing to the rapid quenching to a temperature below the temperature forthe formation of martensite. Moreover, the formation of a relativelythin edge zone during quenching prevents the outer regions of the edgezone from being transformed with a time offset relative to the regionscloser to the core owing to the edge zone being too thick. If individualedge zone regions were to be transformed with such a time offset, thecore zone, which is transformed latest, would produce tensile stressesin the cam track instead of internal compressive stresses. Almostsimultaneous transformation of the cam structure in all regions of theedge zone is thus achieved according to the invention by the setting ofa small edge-zone hardening depth and the immediate quenching of thecam, which can moreover be accomplished by means of oil or by means of acold medium, such as liquid nitrogen. The complete transformation into amartensitic structure that takes place during this process generatesvery high internal compressive stresses in the cam owing to theexpansion in the volume of the thin edge zone due to transformation, andthese stresses overcompensate by far the tensile stresses that resultfrom subsequent joining. In addition, internal compressive stresses atthe surface of the track are produced by shrinkage processes in the camduring spraying.

Expedient developments of the invention can be taken from the subclaims;the invention is moreover explained in greater detail below withreference to an exemplary embodiment illustrated in the drawings; inthese, the FIGURE shows in a cross section a cam produced in accordancewith the invention joined to a hollow shaft.

The FIGURE illustrates a cam 1, which is joined to a hollow shaft 2. Thecam 1, which is composed of the material 100 Cr 6 V HI or C 70 BY HIetc., is produced by hot pressing owing to the speed and economy of thisprocess. However, it can also be formed from a solid piece of materialby mechanical cutting, in particular by milling. A cam can furthermorealso be produced by casting. After hot pressing, the cam 1 is cooled ina controlled manner from the forming heat to give a BY condition or isheat-treated after hot pressing. Overlaps of material on the hot-pressedcam 1 can be removed mechanically if required in order to avoidhardening cracks during subsequent full hardening. After the fullhardening and edge-zone-hardening process according to the invention,the basic 2-stage heating process of which is performed by means of ashaped inductor or annular inductor, it being necessary during thisprocess to shield the tip of the cam in a suitable manner fromoverheating, the cam 1 is thermally stress-relieved by induction or in acontinuous furnace at a temperature below 190° C. This can also beaccomplished in a two-stage inductive process at a frequency in a rangeof from 20 to 40 kHz and a power of about 5 kW. The time required forthe first and second stage is 1-3 seconds, the two stages beingseparated by a pause of 2-6 seconds.

This prevents cracks during the subsequent joining of the cam 1 and thefinal grinding of the cam track 5. During thermal stress relief,internal compressive stresses in the martensite structure are partiallyremoved without transforming the structure itself, as a result of whicha tougher martensite structure is achieved. This also ensures crackresistance during subsequent joining. However, a proportion of theinternal compressive stresses is likewise removed during stress relief;during the stress relief process, care should therefore be taken,through optimized management of the control parameters (e.g. temperingat below 190° C. etc.), that, on the one hand, the internal compressivestresses obtained through edge-zone hardening are not reduced too farand, on the other hand, that the hardness in the edge zone 8 is notreduced too far either, if possible not below 58 HRC.

After stress relief of the edge-zone-hardened cam 1, this is joined insuch a way that high joining stresses act in the base circle 4. Joiningtechniques may vary but the use of internal high-pressure forming is tobe preferred in order to obtain an operationally reliable joint. This isbecause this process involves expanding the hollow shaft 2 plasticallywithout causing the development of heat, the cam 1 being deformedelastically and springing back onto the hollow shaft 2 after the reliefof the high fluid pressure and forming an extremely durable twist-prooffit on it.

As mentioned above, the cam track 5 is edge-zone-hardened in such a waythat the maximum possible internal compressive stresses are built up. Toachieve this, the edge-zone hardening depth must be at most 2.0 mm inthe base circle 4 and in the cam flank 6 and at most 2.2 mm at the camtip 7, more specifically in the middle of the track. To ensure that theedge zone does not penetrate the cam 1 as the roller on the roller camfollower rolls over it, a residual edge-zone hardening depth of at least0.5 mm is required after the finish-grinding of the track 5. With theedge-zone hardening depth thus selected, high internal compressivestresses remain in the base circle 4, even where the joining stressesare up to 250 N/mm². The joining stresses in the cam track 5 aredetermined by radiography. In the cam tip 7, which is not affected bythe joining process, and in the cam flank 6, where there are hardly anyjoining stresses, the internal compressive stresses after joining are atleast 200 N/mm². In the case of concave cam flanks 6, internalcompressive stresses of as much as 300 N/mm² can be achieved afterjoining. The characteristic values given here apply to cams 1 that arestress-relieved at 180° C. after edge-zone hardening and where thegrinding allowance of the cam track 5 is up to 0.7 mm. In the case oflower tempering temperatures or smaller grinding allowances, higherinternal compressive stresses and thus higher permissible joiningstresses can generally be achieved in the base circle 4. With theedge-zone hardening depth subject to its upper limit, it is possible toedge-zone-harden cams with a web height of just 4.0 mm in a manner thatis optimized in terms of internal compressive stresses.

In addition to the possibility of achieving high cam adhesion on thehollow shaft 2 by virtue of maximum permissible joining stressesresulting from the achievement by the invention of high internalcompressive stresses in the cam 1, the cam bore 3 can be blasted withspecial fused alumina, for example, before the joining operation,significantly increasing the adhesion of the cam 1 on the hollow shaft2. If the joining parameters and cam geometries are kept the same, theadhesion achieved is twice as great after such blasting. Using themaximum possible joining stress of 250 N/mm² and the expansion pressureduring the internal high-pressure process, at which the internalcompressive stresses in the base circle 4 of the cam 1 are notcompensated, static torques of up to 400 Nm can be accepted by the camin an operationally reliable manner when blasting is employed.

Finish-grinding of the cam track 5 can be carried out with cubical boronnitride if appropriate. Given adequate cooling, it is even possible toincrease the internal compressive stresses in areas of the cam track 5that are close to the surface to as much as about 600 N/mm² throughappropriate control or setting of the grinding parameters, such as thetype and supply of coolant and the grinding depth, improving thetribological characteristics of the cam track 5. This is advantageousparticularly in the case of cams with a small web height.

What is claimed is:
 1. Process for the production of a cam to be joinedto a hollow shaft to form a camshaft, the cam, which is composed of ahardenable steel, being edge-zone-hardened in a two-stage heatingprocess by induction hardening at a medium frequency of 10-35 kHz to anedge-zone hardening depth of at least 0.5 mm in a middle of a track, andthen being quenched, the material selected for the cam being a steel inwhich the carbon can be dissolved easily as cementite in a pearlite andmartensite or is in the form of a fine composite carbide, wherein afterthe preheating stage of the heating process, the introduction of heat isinterrupted for 0.3-1.5 seconds, and wherein after the action of thepreheat, the cam track is then heated in such a way by means of mediumfrequency that an edge-zone hardening depth of between at least 0.5 mmand at most 2.0 mm relative to the cam base circle and the cam flanksand at most 2.2 mm relative to the cam tip is obtained in the middle ofthe track, after which the cam is sprayed with a quenching mediumimmediately within a matter of milliseconds of the end of the heatingprocess.
 2. Process according to claim 1, wherein the cam is produced byhot pressing.
 3. Process according to claim 2, wherein overlaps ofmaterial on the hot-pressed cam are removed mechanically before fullhardening.
 4. Process according to claim 1, wherein, after edge-zonehardening, the cam is thermally stress-relieved in a furnace at below190° C. or thermally stress-relieved at briefly higher temperatures byinduction.
 5. Method according to claim 1, wherein the cam isheat-treated before edge-zone hardening and, during the tempering thatfollows the heat treatment, a basic hardness of the cam in a range ofbetween 25 and 40 HRC, is established.
 6. Method according to claim 1,wherein the cam is edge-zone-hardened from the BY condition directlyafter hot pressing.
 7. A cam produced according to the process of claim1.